5.3. Results
5.3.1. Physico-chemical Properties
147 5.2.4. Surface Characterization. After finishing the electrochemical experiments, anodes were cut into half pieces, each of which was subject to the physicochemical surface analysis and accelerated life test, respectively. The horizontal or cross-sectional morphology were observed by ZEISS 1550VP field emission scanning electron microscopy (SEM), while elemental compositions were analyzed by Oxford X-Max SDD X-ray energy dispersive spectrometer (EDS) system either in point-and-identification or mapping mode. X-ray diffraction (XRD) profiles were collected using an X’pert MD (Panalytical) diffractometer with Cu-K radiation. Thermo-Scientific Nicolet™ iS™50 Fourier transform infrared spectroscopy (FTIR) was used to characterize surface functionalities. Hetero-junction anodes were held on attenuated total reflectance (ATR) unit in contact either with deionized water or 1 M NaClO4 solutions.
5.2.5. Accelerated Life Test. The stabilities of the Ir0.7Ta0.3Oy/BixTi1-xOz hetero-junction anodes with variable molar fraction of Bi (x) were compared using accelerated life tests.
Each hetero-junction anode sandwiched with a stainless steel cathode (0.5 × 2 cm2, 3 mm distance) was immersed in 1 M NaClO4 solution in a conical tube. Neware Battery Testing System powered the galvanostatic (current density= 1 A cm-2) electrolysis in each test cell. The electrolyte was replaced periodically (3 to 5 h interval), and dissociated amounts of Ti, Bi and Ir in the electrolyte were quantified by Agilent inductivity coupled plasma-mass spectrometry (ICP-MS).
Figure 5.1 Ir0.7Ta0.3Oy/ magnificatio
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Figure 5.2 Ir0.7Ta0.3Oy
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Microscopy ( ction anode
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149
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undergoing the electrochemical experiments. The horizontal morphology of the outer surface appeared to have a cracked and fragmented shapes, where the size of discrete islands appeared to be the lowest for x = 0.1. Observations of vertical cross-section estimated the total depth of mixed metal oxides deposit on Ti support to be approximately 70 μm, while suggested that the depth of cracks is much less than the total film thickness.
The surficial cracks were more distinct for the binary compositions (x = 0.1 ~ 0.7), which presumably ascribed to the differences in thermal expansion coefficients of titanium and bismuth oxides.10 The interface between the underlying Ir0.7Ta0.3Oy and BixTi1-xOz was not distinct perhaps due to inter-diffusions of ions. An EDS mapping image on a cross- section view of Ir0.7Ta0.3Oy/Bi0.3Ti0.7Oz anode (Figure 5.2) indicated, in fact, that thermal inter-diffusion of the metal ions had taken place across the hetero-junction during annealing, on the basis of vertical gradients of Ir and Ta signals.
According to the XRD profiles (Figure 5.3), the hetero-junction anode without Bi doping (x = 0) was characterized by a mixture of anatase and rutile phase TiO2. In the presence of Bi with fractions higher than 10%, the peak from the anatase TiO2 (2θ = 25 º) disappeared. The anatase TiO2 structure has been known to be transformed into a thermodynamically more stable rutile structure by thermal stresses with temperatures higher than 600 ºC.29 However, evidence has been presented that an existence of impurities in the TiO2 lattice structure can significantly reduce the transition temperature;1 the annealing temperature in this study was 425 ºC, which is in the lower range reported by Hanaor and Sorrell29. In addition, Tomita et al.25 reported the crystalline structure of TiO2 prepared from the aqueous titanium-glycolate complexes can
151
Figure 5.3. X-ray diffraction patterns of hetero-junction anodes Ir0.7Ta0.3Oy / BixTi1-xOz with variable molar fraction (x) of Bi (a) referenced with a library for metal oxides of interests (b).
be influenced by the pH of precursor solution. With the current formulation for preparation of the anodes, the process of mixing with the Bi precursor (bismuth citrate dissolved in ammonia) should elevate the mixture precursor pH. In the absence of Ti (Ir0.7Ta0.3Oy/BiOz), the surficial XRD patterns indicate α-phase Bi2O3 and Bi2O4, suggesting that the effective oxidation state of Bi is a mixture of +3 and +5. In this regard, it is well known that monoclinic α-Bi2O3 is converted to cubic δ-Bi2O3 at temperatures
higher than 730 ºC. After cooling the product, metastable β- and γ-phase Bi2O3 are found.23 A visual observation, that the light yellow color of the initial composite as deposited changed into white after the electrochemical experiments, implied that the partial oxidation of Bi (III) to Bi (V) was likely to occur via an anodic potential bias rather than via thermal annealing. For the hetero-junction anodes with Bi fraction from 0.3 to 0.7, the diffraction intensities appeared to be much lower than the other compositional formulations using the same XRD instrument. The relative differences in the effective ionic radii of Bi (103 pm) and Ti (60.5 pm) may result in distortions in the lattice microstructure, which is often observed in binary metal oxide thin film.30
The infrared absorption bands of the hetero-junction anodes in contact with deionized water (Figure 5.4a) were mostly characterized by broad bands at 3000 – 3600 cm-1, relatively sharp peaks centered at 1640 cm-1, and prominent bands starting at around 900 cm-1. Peaks at 1640 cm-1 are usually assigned to bending (deformation) vibrations of free or physi-sorbed water, while the absorption bands ranged from 3000 to 3600 cm-1 are ascribed to stretching vibrations of surface titanol group (>Ti-OH) or hydrogen bonded H2O. Absorption regions at wavenumbers lower than 900 cm-1 occur owing to fundamental vibrations of oxides lattice, particularly from M-O stretching and M-O-M bridging modes (where M is Ti or Bi). The minor peaks at around 2400 cm-1 are attributed to adsorption of carbonaceous species from air. Regarding the Bi-enriched hetero-junction anodes (x = 0.7 and 1), peaks centered near 1400 cm-1 appeared due to hydrated bismuth oxide moieties or surface hydroxyl functionalities (i.e., >Bi-OH) via hydrolysis as in the common case of metal oxides exposed to water (e.g., TiO2, Fe2O3, SiO2, WO3, etc.). The absorption peaks near 3300 cm-1 broadened as the fraction of Bi
153
Figure 5.4. Fourier transform infrared spectroscopy with attenuated total reflectance (FTIR- ATR) analysis of Ir0.73Ta0.27Oy/BixTi1-xOz hetero-junction anodes with variable molar fraction (x); (a) in contact with deionized water, (b) in contact with 1 M NaClO4 solutions, (c) integrated intensities of peaks at 1110 cm-1 in contact with 1 M NaClO4 solutions. Each adsorption spectra was collected with respect to the background signal of the contacting liquid.
increased. In contrast, the intensity of the peak at 1640 cm-1 did not change along with the molar fraction of Bi, presumably due to the interference of interstitial water molecule layers between solid samples and internal reflectance element. Evidence has been presented that the absorption bands in this range (1640 cm-1) originate primarily from coordinated H2O rather than from surficial hydroxyl group.31
According to traditional models describing the surface charge of metal oxides based on their amphoteric nature, the IR absorption from M-OH bonds is expected to decay with the deprotonation of OH functionalities. Therefore, increased isoelectric point of metal oxides at a given solution composition (bulk pH) would allow for losses in absorption by M-OH. In an attempt to assess the role of Bi on the surface charge more clearly, IR spectra were collected in 1 M NaClO4 background solutions with circum- neutral pH. Aside from the absorption profiles described above, characteristic peaks of adsorbed ClO4- appeared at 1110 cm-1 (Figure 5.4b). The integrated intensities of the characteristic peaks monotonically increased along with the molar fraction of Bi (Figure 5.4c). This observation clearly demonstrates that an increase in the surficial Bi fraction is accompanied by an increase in positive surface charges.
5.3.2. Voltammetric Characteristics. Water oxidation on metal oxide anodes is initiated